1. Field of the Invention
The present invention relates to a sheet conveying apparatus conveying a sheet and to an image forming apparatus.
2. Description of the Related Art
Hitherto, an image forming apparatus electro-photographically forming an image transfers a toner image formed in an image forming portion to a sheet fed from a sheet feed portion, guides the sheet to a fixing apparatus, and fixes an unfixed toner on the sheet onto the sheet. It is noted that such image forming apparatus includes what is connected with a sheet processing apparatus to conduct such processes as sorting, stapling, punching, and others on the sheet on which the image has been formed.
By the way, there is a case when the sheet curls (deforms) in fixing the toner image onto the sheet by applying heat and pressure by the fixing apparatus. If the sheet curls, the curled sheet may cause jamming or affect precision of performance of such processes as stacking and alignment. Therefore, it is necessary to correct the curl of the sheet in order to improve quality of the processed sheet.
Then, there is a sheet conveying apparatus conveying a sheet on which a toner image has been fixed to a sheet processing apparatus and is provided with a curl correcting apparatus reversely curling the curled sheet. Some of such curl correcting apparatus includes a first curl correcting portion correcting a curl convexed (projecting upward) with respect to a conveying direction for example and a second curl correcting portion correcting a curl concaved (projecting downward) with respect to the conveying direction. Then, the curl correcting apparatus corrects the curl of the sheet by using at least one of the first and second curl correcting portions corresponding to the directions of the curl.
It is noted that the curl of the sheet occurs because of toner transferred onto the sheet that melts by being heated by the fixing apparatus and then condenses as it is cooled down. Still further, magnitude of the curl varies depending on types of the sheet, temperature and humidity, image density and others. Therefore, in correcting the curl by the curl correcting apparatus, a curl correcting amount to be corrected by the first and second curl correcting portions is determined based on the temperature and humidity, the moisture content of the sheet, the type of the sheet, the thickness of the sheet, the image density, and others.
Normally, each curl correcting portion of the curl correcting apparatus includes a hard roller conveying the sheet and a soft roller in pressure contact with the hard roller. Then, a curved nip portion is formed by causing the hard roller to bite into the soft roller in bringing the soft roller into pressure contact with the hard roller, and the curl is corrected by passing the curled sheet through the curved nip portion.
Here, as disclosed in Japanese Patent Application Laid-open No. Hei. 9-30712, it is possible to adjust the curl correcting amount by changing a pressing force applied to the soft roller to change a bite amount (intrusion amount) of the hard roller into the soft roller. In order to change the bite amount (intrusion amount) as describe above, there is proposed one in which position of one roller is changed by using a motor and a cam member for example.
By the way, one set each of the motor and the cam member is required to independently adjust the curl correcting amount in the first and second curl correcting portions. However, it is preferable to minimize a number of the actuators such as the motors from an aspect of cutting electric power, saving a space, and cutting a cost. To that that, Japanese Patent Application Laid-open No. 2002-93475 proposes a configuration in which the first and second curl correcting portions are independently driven by normal and reverse rotations of a motor by using one normally and reversely rotatable motor and a clutch mechanism, such as a clutch, transmitting a rotational force only in one direction.
Then, in the case of this configuration, driving power of the motor is transmitted to the first curl correcting portion through the clutch mechanism in response to a normal rotation of the motor, and the driving power is transmitted to the second curl correcting portion through the clutch mechanism in response to a reverse rotation of the motor.
By the way, it is essential to be able to control positions of the cam member accurately because the intrusion amount (pressing force) of the roller is adjusted in a plurality of stages for the various parameters such as the temperature and humidity, the moisture content of the sheet, the type of the sheet, the thickness of the sheet, and the image density in the curl correcting apparatus.
However, in the case when the driving power of the motor is transmitted to the first curl correcting portion or the second curl correcting portion through the clutch mechanism, the driving power is not transmitted to one curl correcting portion when the driving power is transmitted to the other curl correcting portion. In this case, the cam member adjusting the intrusion amount of the roller of the curl correcting portion is put into a non-holding condition.
Then if the reaction force to the pressing force of the roller and a disturbance such as vibrations from the image forming apparatus, the sheet conveying apparatus, and the sheet processing apparatus are applied to the cam member in this condition, there is a case when the position of the cam member fluctuates and in such a case, it is unable to accurately adjust the intrusion amount of the roller pair. It is noted that the conventional image forming apparatus includes a sheet conveying apparatus having a plurality of roller pairs besides the sheet conveying apparatus conveying a sheet on which a toner image has been fixed to the sheet processing apparatus. Then, there is such roller pair, provided in the sheet conveying apparatus, configured to adjust a nip pressure by a cam member.
Then, if the reaction force to the pressing force of the roller and a disturbance such as vibrations from the image forming apparatus, the sheet conveying apparatus, and the sheet processing apparatus are applied to the cam member in this condition, there is a case when the position of the cam member fluctuates and in such a case, it is unable to accurately adjust the intrusion amount of the roller pair. It is noted that the conventional image forming apparatus includes a sheet conveying apparatus having a plurality of roller pairs beside the sheet conveying apparatus conveying a sheet on which a toner image has been fixed to the sheet processing apparatus. Then, there is such roller pair, provided in the sheet conveying apparatus, configured to adjust a nip pressure by a cam member.